1. Field of the Invention
The invention pertains to multi band or ultra wide band antennas. More particularly, the invention pertains to multi band or ultra wide band meander antennas.
2. Background of the Invention
Transmitters and transceivers used in wireless communication devices, such as cellular telephones, require antennas of small size and light weight. This is particularly true in connection with portable wireless devices, such as cellular telephones. Many cellular telephones utilize external antennas. Many wireless communication devices must be able to operate over a very wide frequency bandwidth. For instance, in the case of multi-band cellular telephones, they must be able to operate in two or more disparate frequency bands, such as GSM (approximately 900 MHz) and PCS (approximately 1.9 GHz). Accordingly, they must have antennas that are able to transmit and/or receive effectively in both bandwidths.
One simple solution is to provide the telecommunication device with two (or more) separate antennas, each adapted to operate efficiently in one of the given bands. However, this solution is less than ideal because it increases cost, weight, and size of the telecommunications device.
Ultra wide band (UWB) systems also are becoming more and more common. Such systems are used by the military and the public and have extremely wide bandwidths, such as 3-10 GHz or 0.9-6 GHz. Such systems are used, for instance, in high-resolution radar systems. Future military and commercial radios are also expected to have extremely wide bandwidths.
Meander antennas are becoming increasingly popular because they are compact in size, easy to fabricate, light in weight and have omni-directional radiation patterns. A meander antenna can be operated either as a monopole antenna element or as a dipole antenna element depending on the ground plane placement. Meander antennas comprise a folded wire printed on a dielectric substrate such as a printed circuit board (PCB) or a wire wound around a dielectric core. Meander antennas have resonance in a particular frequency band in a much smaller space than many other antenna designs. Typically, the meander antenna element is suspended above or near a ground plane. Generally, the greater the height between the meander antenna element and the ground plane, the wider the bandwidth that can be achieved.
A meander antenna, like many other types of antennas, can be made smaller by employing capacitive loading, and/or dielectric loading. The resonant frequency of a meander antenna decreases as the total wire length of the meander antenna element increases. Also, if the turns in a meander antenna are very close so as to have strong coupling, there can also be capacitive loading of the antenna, which also will increase bandwidth. Total antenna geometry, wire length, and layout can be selected so as to achieve optimal performance for a given antenna. Generally, however, the smaller the meander antenna, the smaller the frequency bandwidth.
Several techniques have been employed in the prior art to increase bandwidth of meander antennas. One technique includes increasing the distance between the meander antenna element and the ground plane.
Another technique is to cascade more than one antenna element, each element being a different size so as to have a different resonant frequency. For example, a feed line on a PCB can terminate in two different meander antenna branches having different frequencies.
A solution along these lines has been proposed in U.S. Pat. No. 6,842,143, which employs two meander antennas of different lengths connected together to cover two frequency bands. U.S. Pat. Nos. 6,642,893 also and 6,351,241 also employ two meander antennas of different lengths connected together. In both, the meander antennas are etched on a flexible dielectric substrate and the substrate is wrapped into a cylindrical shape.
Another technique employed in the past to increase bandwidth is to use a trapezoidal feeding shape, such as disclosed in Shin, Y-S, et al, A Broadband Interior Antenna Of Planar Monopole Type In Handsets, IEEE Antennas And Wireless Propagation Letters, Vol. 4, 2005.
All of these solutions have shortcomings, such as insufficient bandwidth, large volume, higher cost, and/or greater weight.